Adverse environmental conditions as well as physiological situations requiring enhanced secretory protein synthesis can cause an imbalance between demand and capacity of protein synthesis at the endoplasmic reticulum (ER). The ER can sense stress and restore homeostasis by invoking a protective signaling pathway known as the unfolded protein response (UPR). To initiate UPR, yeast largely relies on a linear arm based on the action of a conserved sensor, Ire1p. During the course of evolution, the suite of UPR arms harnessed additional sensors to accommodate more specific responses in a multicellular context. A major challenge in UPR studies is now to understand the biological role of the various UPR arms in intact organisms to define how the UPR signaling network functions to direct diverse cell-fate decisions in development and response to biotic and abiotic stress. The conservation of plant and metazoan UPR and the availability of powerful genomics and molecular tools render the model plant Arabidopsis thaliana an appealing system in which to address these questions. In plants, the UPR is thought to play a major role in various stress situations, but the understanding of plant UPR mechanisms is still limited. In Arabidopsis, two membrane-tethered blip transcription factors, bZIP28 and bZIP60, and a subunit of the conserved heterotrimeric G protein complex, AGB1, have been shown to function as regulators of plant UPR, but little is known about the molecular components of the UPR arms controlled by these mediators. We have recently established a direct involvement of AtIRE1 in ER stress response and in growth. These findings will enable us to explore the role of IRE1 in combination with other key players of plant UPR to develop a model that links UPR signaling during stress responses with growth and development in vivo in a multicellular context. The immediate goals of this proposal therefore are 1) to elucidate conserved and plant-specific features in the signal transduction pathway controlled by AtIRE1; 2) to define the role of the UPR arms in growth, development, and various conditions of stress; and 3) to identify the target genes corresponding to each UPR arm to define downstream effectors and to elucidate how the UPR signaling arms intersect with each other. Adding plants as an evolutionarily distinct and tractable model for the study of the UPR in multicellular organisms is important because it will allow comparing and contrasting plant, yeast, and animal UPRs, and thus will provide significant insights into these systems, adding to our fundamental understanding of eukaryotic cell biology at large. To achieve our goals we will integrate functional genomics with in vitro assays and transcriptomics. We will be able to establish how plants depend on the UPR signaling network during growth and development and in response to stress conditions that require enhanced secretory protein synthesis. Our results will not only enhance our understanding of human growth and disease, they will also permit the development of UPR drugs in a tractable multicellular model, and contribute to our understanding of limiting factors in agricultural processes and plant biotechnology designed to sustain food security on earth.